(1) Field of the Invention
The present invention relates to an image forming apparatus that is orientatable in different positions, and in particular to a technology of detecting the temperature inside a fixing device of the image forming apparatus.
(2) Description of Related Art
When image forming apparatuses such as printers are used, they are usually orientated in a horizontal position so that a surface of the paper feed tray, on which recording sheets are loaded, will be substantially horizontal. This is because such a position prevents the recording sheets on the paper feed tray from bending, and realizes smooth paper feeding.
However, when used in for example an ordinary house with not very large space, it is convenient if the image forming apparatus can be placed in its upright position.
Recently, considering such demand, there have been proposals of an image forming apparatus that can be used in either the horizontal position or the upright position according to the size of the installation space (e.g. Japanese Patent Application Publication No. 8-314333).
However, particularly when an electrophotographic image forming apparatus is used in the upright position, there are possibilities that temperatures of heating rollers of a fixing unit such as a fixing roller cannot be precisely controlled, for the following reasons.
Generally, the temperature of the circumferential surface of the heating roller is detected with a temperature sensor such as a thermistor located near the circumferential surface, without contact with the circumferential surface so as to avoid damaging the surface. On and off of the heating roller is controlled based on the detected temperature so that the temperature of the circumferential surface of the heating roller will be kept at a predetermined level.
However, it should be noted here that such a non-contact temperature sensor is generally configured to detect the temperature of the conductive heat due to the natural convection of the air existing between the heating roller and the temperature sensor. Therefore, when the orientation of the image forming apparatus is changed and accordingly the relative position of the temperature sensor with respect to the heating roller is changed, the convection of the air existing between the heating roller and the temperature sensor might change. Such a change causes a difference in the result of the detection by the temperature sensor.
For example, when the temperature sensor is provided vertically above the rotational center of the heating roller, the detection results will be stable and precise. In contrast, when the temperature sensor is provided beside the rotational center with respect to the horizontal direction, the detection results will vary and will not be very precise.
Therefore, even if the image forming apparatus is designed to appropriately control the temperature of the heating roller based on the results of the detection by the temperature sensor when the image forming apparatus is in the horizontal position, there is a problem that the temperature adjustment does not work properly when the image forming apparatus is in the upright position.
Such a problem can be addressed by modifying correction coefficients, which are used for converting output values from the temperature sensor to surface temperatures of the temperature sensor. In some cases, however, such modification does not suffice to solve the problem, depending on the location of the temperature sensor.